1. Field of Invention
The present invention relates to a global positioning system (GPS), and more particularly, to a method for obtaining a precise intermediate frequency of the GPS.
2. Related Art
A global satellite navigation system is also called a global positioning system (GPS). In the past, the GPS is only limited to military purpose and industrial purpose. With the continuous development of the technology, the GPS begins to be applied to various civil purposes. Generally, the GPS products mainly refer to GPS receivers applied to various purposes, for example, receivers for aviation and voyage purposes, automobile navigation devices, hand-held receivers for mountaineering and entertainment, and other types of communication products. The common GPS product mainly includes an internal antenna, a chipset, external keys, a display panel, and other related parts.
The GPS receiver utilizes satellites orbiting the earth for positioning. Generally, the GPS receiver needs to obtain signals from at least three satellites to calculate the longitude and latitude coordinates of a current position. When the GPS receiver needs to obtain the current position, the GPS receiver compares the time transferred from each satellite. These time differences may inform the GPS receiver of its distance away from each satellite, and therefore the current position is acquired.
A satellite signal received by the GPS receiver includes a pseudo random code and an ephemeris data. The pseudo random code is used to identify a data that the satellite is transferring. The ephemeris data informs the GPS receiver of the positions at which each satellite is supposed to be during the whole day, and therefore the ephemeris data transferred by each satellite presents the orbit data of the satellite. The ephemeris data includes an ephemeris table for recording information continuously transferred by each satellite, such as a state of the satellite, current data, and time.
The process that the GPS receiver searches a satellite to accomplish positioning includes three manners, a cold start, a warm start, and a hot start.
The cold start means that the GPS receiver performs a positioning procedure for the first time after the GPS receiver is turned on. That is to say, no ephemeris data exists in the GPS receiver. At this time, the GPS receiver searches each satellite one by one, and downloads a series of ephemeris data, until all the ephemeris data is obtained and the positioning is completed. Generally speaking, at least 12 minutes is needed for obtaining the ephemeris data completely until the positioning is completed.
The warm start means a normal start procedure, which includes a self-test of the GPS receiver, obtaining an accurate ephemeris data until the positioning is completed. Here, the GPS receiver stores a satellite distribution diagram (i.e., the ephemeris data) for a location of the GPS receiver when it is turned off, so that the satellite distribution diagram is used to predict the ephemeris data at a next warm start, i.e., to predict positions of the satellites in the orbit. Thus, at a next start, the GPS receiver does not need to search the satellite positions one by one, but to predict the positions of the satellites in the orbit presently according to the ephemeris data recorded when the GPS receiver is turned off last time. However, the predicted ephemeris data is not accurate and may not be used for the positioning. Generally speaking, the time from the warm start till the completion of the re-positioning is 40 seconds.
The hot start means a procedure that the GPS receiver is turned on after it has been turned off for a short time, or means that the GPS receiver needs to acquire a part of the ephemeris data again to accomplish the positioning when the received satellite signal is poor. Generally speaking, the time from the hot start till the completion of the re-positioning is 10-20 seconds.
Thus, no matter how the GPS receiver is started, the GPS receiver may have no ephemeris data, or may have an incorrect ephemeris data due to incorrect time or position, and thus an ephemeris data needs to be obtained again for re-positioning.
In addition, the GPS receiver in a sleep state needs to obtain the positioning data again. When a user has not given any instruction to the GPS receiver for a period of time, the GPS receiver enters the sleep state automatically. When the user needs to perform navigation again, the ephemeris data is not correct due to that a considerable time has passed since the GPS receiver performs navigation last time, or that the current location is too far away from the location for the navigation last time, and thus an ephemeris data has to be obtained again for re-positioning.
The GPS receiver uses an oscillator as a reference frequency source required during operation. In the prior art, the oscillator used by the GPS receiver is usually a high-precision oscillator that is adjusted and calibrated to a particular frequency, such as a temperature compensated crystal oscillator (TCXO).
However, as the oscillator is subjected to different manufacturing qualities, an actually generated intermediate frequency is usually inconsistent with a designated intermediate frequency of the oscillator given in a specification of the oscillator. Thus, when the satellite is being searched, the satellite may be searched in a nearby frequency range according to the designated intermediate frequency of the oscillator. However, as an error exists between the actually generated intermediate frequency of the oscillator and the designated intermediate frequency, usually a large range is required for searching the satellite. In such a manner, the GPS receiver has to spend considerably long time on the preliminary satellite searching.